Friday wrap-up: LIGO, chasing the 750 GeV excess...

Apologies for the long hiatus -- other aspects of life have been getting in the way. Here is a summary of the last month or so...

• Of course the biggest news was the first observation of gravitational waves, a binary stellar-mass black hole system, and a binary black hole merger. Not bad for an 8 page paper! The signal is really quite striking; it's wonderful to see the agreement between the two detectors. I reproduce the observation plot below, just because one cannot admire it enough.
  
  
  
  
  
  Interest in the finding was phenomenal; the Physical Review Letters server even crashed (they were getting 10k hits per minute). One can find plenty of explanations at various levels online: e.g. for the layperson see Quanta, or Brian Greene on the Late Show; for the more scientifically minded there exists a digestible summary of each paper by Christopher Berry; or for a colloquium-level talk see Barry Barish at CERN. Lastly you can enjoy the xkcd.
  
  Here I just want to mention some interesting facts, taking as read the core ideas behind the phenomenon and the measurement. The event was actually observed before the first planned science run, during an engineering run. It was identified within 3 minutes and the decision was subsequently made to keep settings in place to take 16 more live days of data. This time period was chosen so that the data-driven background estimation could nail down the unlikeliness of the event to >5.1σ under the background-only hypothesis. Below is shown the event and background estimation; the detection is well in excess of 5.1σ, even including the event itself in the background estimation.
  
  
  
  
  The data is in fact completely open and you could analyse it yourself! In addition to the GW150914 event there are also two others that rise somewhat above the background ("GW151012" and "GW151226"). You can see them by eye in the above plot. They are clearly not statistically significant enough to announce a discovery alone, but still they are tantalising... with room for improvement to design sensitivity (by a factor of ~2 which increases the spatial reach by 2^3) and the construction of a third detector in India to triangulate the signal, the future of gravitational wave astronomy is exciting.

• There's also that puzzling observation by Fermi of a gamma-ray burst 0.4s after the gravitational wave detection. There are good reasons for and against believing this was associated with the GW150914 event (see Quanta); the best way to tell is to just to wait and see if it happens again!

• On the 750 GeV diphoton excess you can read Jester's "750 ways to leave your lover" on various explanations.
  
  As well, a comprehensive paper appeared on the arXiv reviewing some of the renormalisable and weakly-coupled explanations. In the authors' literature review they "found a wide range of mistakes or unjustified assumptions, which represent the main motivation that prompted this work." The suggestion is to utilise computational tools (e.g. SARAH) to automate the work which the phenomenologist should be doing anyway for a thorough analysis, and they provide 40 model files to match models already in the literature.
  
  Let us review these "mistakes or unjustified assumptions"; we will refer to the resonant 750 GeV state as $S$ throughout, and consider models where the effective coupling of $S$ to the diphoton/digluon vertices are induced by a loop of gauge-charged fermion(s) or scalar(s) [of course there are explanations which do not fit into this framework]...
  
  Next-to-leading-order (NLO) corrections to the $S$ decay widths matter. Compared to the LO result used in many papers, NLO corrections typically decrease $\Gamma(S\to\gamma\gamma)$ by O(10%), and N3LO corrections can increase $\Gamma(S\to gg)$ by a factor of almost 2. Overall this means $Br(S\to gg)/Br(S\to\gamma\gamma)$ is typically underestimated (for scalar $S$) by as much as a factor of 2 when using the LO estimate. This will change best fit regions and lead to stronger constraints from the dijet channel. Models which live on the edge of exclusion based on LO estimations may not survive.
  
  It is often assumed that $S$ does not mix with the SM Higgs. But mixing is necessarily generated at some loop level. If a fermion is in the loop it is a three-loop effect (but with large Yukawas and strong gauge couplings at the vertices). If a scalar is in the loop it arises at one-loop. This contribution can be turned off by tuning a quartic term to zero [this is not stable under the renormalisation group evolution], but there is always a pure-gauge two-loop contribution. This effect should be acknowledged and checked for consistency.
  
  Another common assumption is that the $S$ vev is zero. But since there is a $Sgg$ vertex the $S\to -S$ symmetry must be broken when expanded around the vacuum ($S=S_0 - \langle S_0 \rangle$). It is hard to imagine a non-finely-tuned potential with this property and a minimum at $\langle S_0 \rangle=0$. Another way to argue this is made in the paper: if the original state $S_0$ couples at a three-point vertex with a new fermion or scalar, then a tadpole term will induce a non-zero linear $S_0$ contribution which acts like a vev insertion.
  
  Decay channels have been missed in some works which can significantly change conclusions.
  
  In the proposed models it is necessary to have a rather large diphoton width. The authors identify three main methods for achieving this. There are worries with each of them which have not been addressed uniformly in the literature....
  
  1. If fermion in the loop, then a large Yukawa coupling $yS\psi\bar{\psi}$. Typically these need to be O(1). Naturally, one should make sure perturbativity is under control when calculating the one-loop effective coupling. There exist papers which don't. As well, even if it remains pertubatively controlled at the 750 GeV scale, the renormalisation group evolution can evolve that Yukawa to the non-perturbative regime at some higher energy. This should be checked and at least acknowledged. [It is an interesting fact that this does not happen in the standard model for the top Yukawa; it is ~1 at the electroweak scale and shrinks with energy scale due to Higgs/top quark gauge contributions].
  
  2. If scalar in the loop, then a large cubic term $\kappa S XX$. The authors point out that this generally leads to problems with stability of the electroweak vacuum. I will also add that if the cubic term is large (>TeV) compared to the desired sub-TeV particles, then it is likely that the vacuum potential must be somewhat tuned, and this will not be stable under radiative corrections.
  
  3. Instead of relying on a large Yukawa or cubic, increase the charge coupling the loop fermion/scalar to gluons/photons or have more particles in the loop. Papers exist with Q≥5 and N=9000. However, such changes induce a large correction to the gauge coupling renormalisation group running above threshold, and can lead to high-energy electroweak behaviour which is ruled out, or worse to a Landau Pole at energies below 1 TeV.

• My conclusion from all this: even if one does not object to the phenomena of ambulance chasing [I personally do not object to the principle], one should object to the lack of quality that seems to go along with it. It is a problem that assumptions are made and effects are overlooked which change conclusions considerably. It is a problem that lower quality papers are (at least for a good while) cited on par with better considered ones. It is a problem that we are mostly seeing the same idea embedded into different models with no qualitatively new observations. In addition, it is frustrating that (non-)participation in trending topics has implications for whether you can continue to make your way in the field, especially for early career researchers.

• On a lighter note, see the arXiv preprint "A Theory of Ambulance Chasing" by Mihailo Backović (a 750 GeV ambulance chaser himself!) for a bit of fun, where he attempts to model the total number of papers on a trending topic as a function of time. For the diphoton excess: "It follows that if the interest scales as an inverse power law in time, the cumulative number of papers on a topic is well described by a di-gamma function, with a distinct logarithmic behavior at large times." "Di-gamma" is just brilliant. A (testable) prediction of this model is that "the total number of papers will not exceed 310 by June 1, 2016". If you feel like you have a better model, then throw your hat into the ring! 

• The HEP Postdoc Project has appeared. To quote the website: "The HEP Postdoc Project intends to be a tool for Postdocs, or even PhD students, in the area of High Energy Physics... When an applicant accepts an offer, she/he is lacking, however, important information about the senior researchers in the corresponding institution... The goal of the HEP Postdoc Project is to fill this gap. Please, send us your opinions on senior high energy physicists you have interacted with in the past..."

• CERN is doing an "In Theory" series of articles on the CERN Theory department. The first-two installments are "Welcome to the Theory corridor" and "why bother with theoretical physics?"

• Conferences/workshops:
• CoEPP Annual Workshop 2016 (indico)
• LHC Performance Workshop (indico)
• UCLA Dark Matter 2016 (agenda)
• CERN Winter School on Supergravity, Strings, and Gauge Theory 2016 (indico)

• Links without thinks:
• Stories of Australian Science: Looking for dark matter in a gold mine.
• Sabine Hossenfelder via aeon: The superfluid Universe.
• SciAm: Physicist Sabine Hossenfelder Fears Theorists, Lacking Data, May Succumb to "Wishful Thinking".
• Smashpipe: Who's winning the string wars and why should you care? [Part 1 and Part 2]
• Quanta: From Einstein’s Theory to Gravity’s Chirp.
• Lawrence Krauss via New Yorker: Do the New, Big-Money Science Prizes Work?
• symmetry magazine: The ABCs of particle physics.

• In audio/video media:
• Recordings of talks from "Why Trust a Theory? Reconsidering Scientific Methodology in Light of Modern Physics" [In my opinion, more conferences/workshops should record and make public their talks like this].
• CMS Experiment: An introduction to the CMS Experiment at CERN. [7:25]
• Katherine Freese at Perimeter Institute: The Dark Side of the Universe. [1:03:16]
• Gianfranco Bertone: The Quest for Dark Matter. [1:00:23]
• Camilo Garcia-Cely: Phenomenology of Left-Right Symmetric Dark Matter. [1:06:30]
• Gero von Gersdoff: Light by light scattering and the 750 GeV diphoton excess. [58:00]
• The Good Stuff: What the Heck is Dark Matter? [12:02]
• Stephen Sekula SMU Godbey Lecture: "The Tail of the Lion: 100 Years of General Relativity, the Scientific Theory of Space and Time" [1:10:05]
• La physique autrement: Physics and caffeine. [9:12]

Friday wrap-up: diphoton uncertainties, dark matter uncertainties...

Wherein I list some (mostly) recent happenings, ramble a bit, and provide links, in an order roughly determined by importance and relevance to particle physics. Views are my own. Content very definitely skewed by my own leanings and by papers getting coverage, and it may not even be correct. It is a blog after all...

• There's quite a bit of discussion over at Résonaances (see also the comments) surrounding the Davis-Fairbairn-Heal-Tunney paper proposing an underestimated systematic in the background parameterisation used in the ATLAS diphoton analysis. This (and related) discussion looks to have aided (according to the acknowledgments) the preparation of another paper from Bradley Kavanagh, which seems to clarify the issue. In that paper it is written:
  
  Davis et al. introduce a different possible parametrisation for the background (which was also validated by a Monte Carlo study) and find that the significance of the excess is further reduced with respect to the k = 1, fixed-N case. However, the empty bins at high mγγ were not included in that analysis, leading to a background fit which overestimates the high mγγ event rate. Indeed, using the Davis et al. background parametrisation (with free normalisation) in this analysis gives a local significance of 3.8σ for a free-width resonance. This does not discount the possibility that exploring a wider range of possible background functions may impact the significance of the 750 GeV excess, but the correct constraints from the entire range of mγγ should be taken into account.

• A few-interesting-papers appeared concerning baryonic effects on the local dark matter velocity distribution, of interest for interpreting direct detection experiments (see Matthew Buckley's blog for a write-up of one of them). Each of the papers takes a number of simulated Milky Way-like galaxies and looks at the dark matter distribution at Solar radius. Naturally, due to the small number of simulated galaxies, the papers reach slightly different conclusions. What is clear, though, is that there are significant uncertainties in both the local density and the local velocity distribution, which means that the usual direct detection limits you see drawn on e.g. σSI versus mχ space should be taken with a small grain of salt, since they assume the standard halo model. Also of note is that these effects alone cannot ameliorate tension with the DAMA/CoGeNT events. Further work in this area will be interesting to follow as additional (and more detailed) simulations become available.

• Links without thinks
• .Mic: "With One Hashtag, Female Astronomers Share Their Heartbreaking Stories of Harassment"
• Nicolas Gisin via IQOQI: "Thought police – on arXiv?"
• BackReaction: "Does the arXiv censor submissions?"
• nature: "Hawking’s latest black-hole paper splits physicists"
• Ars Technica: "The search for dark matter heats up"

• In audio/video media:
• IQIM Caltech: Anyone Can Quantum, a quantum chess match between Paul Rudd and Stephen Hawking. [11:50]
• CERN: CERN's new microcosm exhibition is now open. [0:41]
• Fermilab: Quantum Field Theory. [5:29]
• Fermilab: Why I Love Neutrinos - Professor Naba Mondal. [0:59]

• A sad day for Comic Sans enthusiasts everywhere (nowhere?) -- apparently no more from Fabiola...
  
  

  Historical moment announced at @CERN during DG Fabiola Gianotti's new year address: The End of the #ComicSans Era! pic.twitter.com/nhw6TabNbZ

  — Steven Goldfarb (@stevengoldfarb) January 18, 2016

Friday wrap-up: diphoton excess, no diboson, no gluinos...

What a week! We have already seen some 40-odd papers submitted to hep-ph in the last few days on the "recent observed diphoton resonance" [1]. Well I certainly wouldn't go that far but ATLAS and CMS have each seen an excess of events in the diphoton spectrum at around 750 GeV, which is amazing since apparently they weren't even searching for it [2], and anyway beside the point because they also discovered a gluino [3]. Sloppy science writing aside, what do we know?...

• The CMS and ATLAS Run II physics results presentations can be found here. Of course, all results presented are preliminary. The result that has hep-ph buzzing, though, is a little bump atop the falling diphoton invariant mass background (conference notes here and here). [See Jester, Motl, Strassler (here and here), PhysicsMatt, or Eilam Gross for some physicist perspectives. Else in popular media I thought the NY Times article was fairly balanced, but then I am a phenomenologist]. You can eyeball the bumps in question below (credit to Strassler for this image):
  
  
  
  
  But what about the numbers? The rumours were as accurate as one could reasonably ask: assuming a narrow width resonance, CMS observed a 2.6σ local (1.2σ global) excess at 760 GeV [increases to 3.0σ local (1.7σ global) at 750 GeV when combined with the 8 TeV data], and; ATLAS observed 3.6σ local (2.0σ global) at 750 GeV [have not yet combined with 8 TeV, but if they did it appears the significance would fall]. Allowing the width to float to larger values, the CMS result goes down to 2.0σ local, whereas ATLAS observes a best fit 45 GeV (6%) width at 3.9σ local (2.3σ with multivariate look-elsewhere). The relevant slides are below:
  
  
  
  
  
  It is a tantalizing excess. Sensibly, what one would like to know is the global significance of the fully combined (CMS+ATLAS 8+13 TeV) datasets. It is non-trivial to get an exact number (see here or here), but one can at least make a good bet that it's greater than about $\sim \sqrt{1.7^2+2.0^2}\approx 2.6\sigma$, perhaps in the vicinity of $\sim 3\sigma$. [I would imagine the demand for a joint analysis is high enough to be a priority for the collaborations (or they might try to avoid feeding the hep-ph sharks?), so maybe we will have that number by Moriond]. This being a (very rough!) ~1/300 chance then, and given the hundreds of plots CMS and ATLAS produce, it is very possible that this is just a statistical fluctuation. Nonetheless, this excess is being taken fairly seriously, and will be exercising our scrolling finger on hep-ph for the foreseeable future while we grapple with the sensible question: if it is real, then what could it be and what does it imply? The answer to this question may have implications for the experimental program of the LHC over the next few years (at least), and so phenomenologists are already relentlessly hard at work...
  
  So let's try to answer that question: what could it be? Well, there is no evidence for any extra activity in the excess events, so it appears consistent with a simple $gg\to X\to \gamma\gamma$ resonance. If taken as a resonance, the events translate to a cross-section $\sigma(pp\to X)\times Br(X\to \gamma\gamma)$ of $\sim 2$/fb ($\sim 6$/fb) in the narrow (wide) width scenario. Let us try to build a model with these properties. The simplest thing is to add a scalar singlet to the standard model. To couple it to gluons and photons let's borrow the Higgs' trick and couple it to some coloured/charged fermion(s) which then induce the couplings via a loop. Let's try Yukawa coupling it to a vector-like up-type quark first, write down the effective couplings, and calculate the Yukawa necessary; we find that it has to be huge ($\sim 5$ or so). And there's a potential problem, since the singlet will want to decay most of the time to the up-type quark. That's okay! We will just make it heavy enough (> 375 GeV) so that it's not allowed. Now we're done, and this solution is "already well-known" [4]. We can add more vector-like fermions to quell the large Yukawa(s) somewhat and/or dial the $gg$ and $\gamma\gamma$ couplings independently. If we take the large width seriously, we still have to add extra decay channels, and then dial up the production and/or branching to photons to compensate. The obvious options are a dark sector or some other standard model states, which we have to hide from previous searches. We could also try constraining ourselves inside some more predictive (restrictive) model.
  
  Of course there are several papers on just the above, the implication being that you need more than just the singlet scalar, which is obviously quite interesting. The immediate implications for the LHC are: look for anything at 750 GeV in $jj, Z\gamma, ZZ$ (in roughly descending order of promise) as soon as is possible.
  
  But this is just a minimal model. It could also be a scalar/pseudoscalar/bound state connected to compositeness/extended gauge group/extra dimensions/hidden valley/SUSY/dark matter/naturalness, and you can be sure there are already arXiv submissions on all of these. On that, it seems to me that arXiv isn't quite the ideal platform for all this. It would be nice instead to have all the various proposals in the same place, with the same formatting, in no-nonsense form, all grouped by some general properties. Then the interested phenomenologist/experimentalist could go and browse a list of, for example: (1) candidate; (2) production; (3) couplings; (4) decays; (5) associated activity; (6) additional particles; (7) additional predictions. Of course this will inevitably be done anyway by some authors in a review, but it seems like the same could be achieved much more efficiently with a community-run wiki or similar, as long as there were some moderators willing to dedicate their time to such a project... any thoughts on this from readers?
  
  In my book there's not much more to say except we need more data, to tell (1) if this is real, or (2) what it is. Looking forward to more excellent work from our experimental colleagues in the new year.
  
  
  
• Now onto other matters from the presentation. First the diboson excess from Run I. Before the meeting a couple of useful papers appeared on the arXiv: a third-party CMS+ATLAS statistical combination, and; a thorough summary and literature survey. Now we know both CMS and ATLAS see nothing significant in Run II data (although they do not have sensitivity to conclusively probe the parameter space of interest):
  
  
  
  

• Also in Run II data, the on-Z excess is not seen by CMS, but still persists at ATLAS...
  
  
  

• As well, lots of gluino searches in different final states but nothing seen, and limits improve to roughly 1.2--1.8 TeV in the simplified models considered (but of course there are always compressed places to hide!).

• CMS have not unblinded any of their Higgs analyses, but ATLAS reported results in γγ and ZZ: they were expecting 3.4σ observation and saw instead 1.4σ. Obviously the Higgs has packed up, moved to 750 GeV, and remembered its earlier proclivity for photons (this hypothesis will be robustly tested in upcoming LHC analyses).

• Moving on to other news, LUX has released new limits on spin-independent dark matter nucleon scattering. See the press release and/or this blog post from Sally Shaw for a summary. They're almost observing solar neutrinos!
  
  
  

• "NuPhys2015: Prospects in Neutrino Physics" was on this week (indico).

• Winners of the 2015 InterActions Physics Photowalk were announced.

• Links without thinks:
• Strumia's insta-paper archive.
• Quanta: "A Fight For the Soul of Science," on the recent meeting at the intersection of the philosophy of science and theoretical physics.
• Quanta: "Landmark Algorithm Breaks 30-Year Impasse."

• In audio/video media:
• Fermilab: "Particle Physics [teenager style]." [4:55]
• SixtySymbols: "The Force." [13:40]
• In a Nutshell: "Black Holes Explained - From Birth to Death." [5:55]
• MinutePhysics: "The Physics of Car Crashes." [3:09]

[1] arXiv: "The recent observed diphoton resonance around 750 GeV at the LHC..."

[2] Nature News: "... the 750 GeV boson is not one of the particles that LHC physicists have been searching for..."

[3] Tech Times: "Physicists Have Discovered Evidence Of A Gluino Particle, The Cousin Of The Higgs Boson."
[4] arXiv: "It is already well-known that a real singlet scalar ϕ with Yukawa couplings ϕXX to vector-like fermions X with mass mX>mϕ/2 can easily explain the observed signal, provided X carries both SM color and electric charge."

Friday wrap-up: diboson update, XMASS...

Wherein I list some (mostly) recent happenings, ramble a bit, and provide links, in an order roughly determined by importance and relevance to particle physics. Views are my own. Content very definitely skewed by my own leanings and by papers getting coverage, and it may not even be correct. It is a blog after all...

• It's the season for conferences! This week we have...
• 8th International Workshop on Top Quark Physics (TOP2015: indico; twitter). One of the interesting new results includes evidence for (>3σ) single top quark production in the s-channel with the 8 TeV dataset. There's an entertaining review of the first two days here from James Howarth.
• Particle Astrophysics and Cosmology Including Fundamental InteraCtions (PACIFIC 2015: agenda).
• Corfu Summer Institute: 15th Hellenic School and Workshops on Elementary Particle Physics and Gravity (programme).

• A couple of interesting conf-notes out this week as well. The first is the ATLAS/CMS combined Higgs results we saw presented a couple of weeks ago.

• The second is an ATLAS diboson resonance search which combines the results from the large-R dijet channel with the leptonic channels. The results are well summed up by the first Figure in the Appendix:
  
  
  
  
  
  In short, when interpreted as a $W'$ resonance decaying to $WZ$, they see a 3.4σ local excess in the boosted jet topology and absolutely nothing in the leptonic channels. As well, these leptonic channels were sensitive to the $W'$ interpretation of the dijet excess, so that the local significance when combined falls to 2.5σ. Taken at face value then, if the dijet excess is really new physics, it is unlikely to be as simple as $W'\to WZ$. [As an aside: I do wonder how the community's reaction would have differed if this were that paper that was published first?]. To mimic such a signal without the leptons you would need a heavy resonance decaying to two exotic particles with mass $\sim m_Z$, which then decay mostly to quarks... would be difficult to hide these low mass exotics. Or else it is something more complex that happens to pass the selection criteria for the fat jet analysis but produces very few isolated leptons. Anyway, there have been >30 extra citations to the original ATLAS paper since I made a quick literature survey seven weeks ago, and more every week. For me it seems sensible to just wait and see what the new data says (probably some time next year), happy to watch the ambulance in the distance, starting to speed up...

• The TAUP2015 parallel session slides are now up. Indeed, as speculated last week, XMASS has a best fit modulation that is opposite in phase to that seen by DAMA/LIBRA (see Slide 10 [pdf]). It is enough evidence to exclude much of the region where the DAMA signal can be interpreted as a standard WIMP with spin-independent nucleon scattering cross-section (though this is nothing new). Interesting to see what their results will be in the fiducial volume (analysis ongoing).
  
  
  
  
  
• 32 Australian institutions have signed up to the Science in Australia Gender Equity (SAGE) pilot: "Commencing in September 2015, the pilot requires participants to collect, analyse and present data on gender equity policies and practices in STEM departments, as well as identify gaps and opportunities for improvement."

• Links without thinks:
• Institute for Advanced Study: "Beyond the Higgs: From the LHC to China."
• Richard Dawid wrote a guest blog on the reference frame: "What confirms a physical theory?" This should be taken in the context of that Ellis/Silk nature comment article and the ensuing debate on post-empirical science.
• New Scientist: "Black holes may be brick walls that bounce information back out." On 't Hooft's new contribution to the black hole information paradox...
• ... and Sabine Hossenfelder's reaction at Starts With a Bang: "Black holes and academic walls."
• Also at Starts With a Bang: "Will The LHC Be The End Of Experimental Particle Physics?"

• In video/audio media:
• CERN: A superfast chip for particle detection. [3:34]
• MinutePhysics: Why It's Impossible to Tune a Piano. [4:19]
• Vi Hart: Infinite Trees Are Super Weird. [6:18]
• Numberphile: Langton's Ant. [4:28]

• Lastly, in images from space, it is hard to top these new images of Pluto!
  
  
  
  
  

Friday wrap-up: diboson excess, EPS-HEP, XENON100...

Wherein I list some (mostly) recent happenings, ramble a bit, and provide links, in an order roughly determined by importance and relevance to particle physics. Views are my own. Content very definitely skewed by my own leanings and by papers getting coverage, and it may not even be correct. It is a blog after all...

I am back from a six week tour of Europe (Greece for Planck conference, UK for seminar talks, Italy for ICTP Summer School and talk in Rome) followed by a proper two week holiday (Hawai'i for lava and turtles)... hence the inactivity here. In my absence, the diboson excess has been hot, the first 13 TeV results have been already trickling out, and in other good news it is 92% probable we are even doing something "worthwhile" -- hey, that's almost 2σ!

Let me attempt an incomplete summary of the last month...

• It's been almost two months now since the ATLAS diboson excess hit the arXiv (see Resonaances for a description), and many theorists/phenomenologists have now had the time to digest and interpret the result. The paper has been cited 37 times, and I count 31 dedicated studies. Let's take a stroll through them in the hopes of learning (in some Bayesian sense -- of course you will have to unweight for selection bias) what is a likely explanation if the signal persists... [This is only a quick survey and probably not completely accurate; send me a message or leave a comment if you believe I've done any of these papers a grave injustice...]
  
  

  Paper	Authors	Candidate	Comment
  1507.07406	Faraggi, Guzzi	$Z', W'$	String inspired GUTs
  1507.07102	Lane, Prichett	$\rho, a_1$	Vector or axial triplet in composite Higgs
  1507.06499	Fritzsch	$Z^*, W^*$	Excited states of composite weak bosons
  1507.06312	Kim et al.	-	EFT study
  1507.06018	Bian et al.	$\rho$	Vector triplet in composite Higgs
  1507.05299	Anchordoqui et al.	$Z'$	Leptophobic, string inspired
  1507.05310	Chao	$H$	2HDM
  1507.05028	Omura et al.	$H$	2HDM
  1507.04431	Chen, Nomura	$H, H^\pm$	2HDM
  1507.03553	Sanz	Exotic glueballs	Perhaps within composite Higgs framework
  1507.03428	Fukano et al.	Dilaton	e.g. scale-invariant generic heavy vector triplet model
  1507.03098	Cacciapaglia et al.	Pseudoscalar	Weak singlet with Wess-Zumino-Witten (effective) couplings
  1507.02483	Chiang et al.	Composite Spin-0	Hidden confining gauge theory coupled to SM via D5 operators
  1507.01923	Dobrescu, Liu	$W'$	$SU(2)_L\times SU(2)_R\times U(1)_{B-L}$ model
  1507.01638	Allanach et al.	$Z', W'$	(motivated by EFT) within $SU(2)_L$ or $SU(2)_R$ vector triplet
  1507.01914	Carmona et al.	Vector resonances	Composite Higgs (non-custodial)
  1507.01681	Abe et al.	Vector resonances	Partially composite [G221 model with one dynamical SU(2)]
  1507.01584	Heeck, Patra	$W_R$	$SU(2)_L\times SU(2)_R\times U(1)_{B-L}$
  1507.01185	Abe et al.	$Z', W'$	G(221) 'three site moose model' e.g. KK excitations of weak bosons
  1507.00900	Cacciapaglia, Frandsen	-	Unitarity study
  1507.00268	Cao et al.	$Z', W'$	In G221 and G331 models
  1507.00013	Brehmer et al.	$W_R$	$SU(2)_L\times SU(2)_R\times U(1)'$
  1506.08688	Thamm et al.	Composite $Z', W'$	Within vector triplet
  1506.07511	Gao et al.	$W_R$	$SU(2)_L\times SU(2)_R\times U(1)_{B-L}$
  1506.06767	Alves et al.	$Z'$	$U(1)_{d-u}$
  1506.06739	Aguilar-Saavedra	$(VVX)$	Triboson final state mimicking a VV resonance
  1506.06736	Dobrescu, Liu	$W'$	$SU(2)_L\times SU(2)_R\times U(1)_{B-L}$ model
  1506.06064	Cheung et al.	$W'$	$SU(2)_L\times SU(2)_R\times U(1)'$
  1506.04392	Franzosi et al.	Composite $Z', W'$	Within vector triplet
  1506.03931	Hisano et al.	$Z'$	Leptophobic
  1506.03751	Fukano et al.	Technirho	Vector triplet within walking technicolour (composite) model

  
  Looks like the most popular explanation is a $W'$ within an extra vector triplet, either arising from an extended gauge sector (minimally a G221 model) or as a low-lying composite state. Less popular, but still well represented, are explanations via a leptophobic $Z'$ or a heavy Higgs in a 2HDM with the second Higgs doublet coupling strongly to the first generation quarks. A notable absence is any (minimal) SUSY explanation.
  
  Many (but certainly not all) of these models tend to predict observable $WZ$ and $WW$ resonances ($ZZ$ is difficult for a spin-1 due to Landau-Yang), usually in conjunction with $Wh$ (just by naive equivalence theorem). These are channels to keep an eye on during Run II.

• The first 13 TeV results are already being released! E.g. check out all-these ATLAS notes which have appeared in the last couple of weeks (just in time for EPS-HEP). For the record, CMS had the first as far as I know (charged hadron pseudorapidity distributions).
  
  In particular, ATLAS released a plot (below) which beautifully agrees with the standard model as per usual: top quark pairs at 13 TeV just where they're supposed to be!
  
  
  
  
  
• The EPS-HEP conference ran this week from 22-29 July. The slides are available on Indico here. I was impressed by the live webcast of plenary sessions, the daily newsletters, and the well-used hashtag which almost made it possible to attend the whole conference online. Some highlights for me...

• LHCb presented preliminary results in their search for long-lived light scalars (see this talk [pdf] from Andrea Mauri) in $B\to K^* s \to K^*(\mu^+\mu^-)_{displaced}$ decays; they see no significant signal above background. Last year I gave a talk to the LHCb rare decays group motivating such a search, so it is very exciting to now see results! Below are the limits they set on the $B$ meson branching fraction for different lifetimes.
  
  
  
  
  
  The simplest model which can give this phenomenology is the standard model plus a real singlet scalar (Higgs portal), as described in an earlier post here. The pertinent free parameters of that model are the light scalar mass and a mixing parameter, and this new result will constrain that parameter space. To get a feel for how much, I picked off the limit lines (sans the statistical fluctuations which can be scraped from the vector plot once the preprint is out) and translated them. [Here I am taking data from an unpublished plot presented at a conference... have I learned nothing from BICEP?] Anyway, the exclusion result is shown in orange in the following figure (the grey shaded regions indicate lifetimes of 0.1mm, 1mm, 1cm,... for more details on the plot see here):
  
  
  
  
  Interestingly, LHCb competes with the BaBar exclusion curve (grey) even for very low masses. It was not obvious at all that LHCb would be able to do this, since for these low masses the long-lived light scalars are very boosted and many will escape their detector. Looking forward to reading the preprint when it is out!

• Two months ago we mentioned the new LHCb result on $R(D^*)=Br(B\to D^*\tau\nu)/Br(B\to D^*\mu\nu)$. The heavy flavour averaging group (HFAG) have now released their combination average; it's 3.9σ from the SM. (See talk from Marta Calvi [pdf]).

• LHCb published in Nature Physics their exclusive measurement of $|V_{ub}|$ in $\Lambda_b$ decays, an important result in resolving the $V_{ub}$ puzzle. You can read the LHCb release here. It has been on the arXiv since April, so it's not a "hot off the press" result, nevertheless it is now for some reason being picked up by various news sources as a blow for supersymmetry (see-these-four-examples). Good to know that if we see nothing in LHC Run II there is at least one way to sell the null result to the media... even though as a scientist such a result would be extremely interesting!

• LHCb have claimed the discovery of pentaquarks (paper here and EPS-HEP slides from Sheldon Stone here [pdf]), a $J/\psi p$ resonance in $\Lambda_b\to J/\psi p K$ decays.
  
  
  
  
  
  This comes 12 years after SPring-8 first announced (the later ruled out) evidence for such states. One cool thing about the LHCb result is that you can even see it by eye in the Dalitz plot (below as line in $m^2_{J/\psi p}$); the LHCb team cannot account for it with any known $\Lambda^*$ resonance or interference. The best fit is in fact found by including two new $uudc\bar{c}$ pentaquark states.
  
  
  
  
  There's a good Quantum Diaries post from Adam Davis about it here (see also nature news, symmetry, Jon Butterworth, and Tommaso Dorigo + comments).

• This week the XENON Collaboration released an arXiv paper, "Search for Event Rate Modulation in XENON100 Electronic Recoil Data". They see a 2.8σ annual modulation signal in low energy single scatterings with a phase consistent with DAMA/LIBRA (!) ... and then pour a serious amount of cold water on the measurement. In order of decreasing temperature, here are the buckets they use: (1) There is no globally significant modulation in the data. (2) The phase of the annual modulation signal deviates from that expected for a standard dark matter halo by 2.5σ. (3) The amplitude is much lower than that expected if DAMA/LIBRA was correct. (4) A 2.5σ annual modulation signal is seen in low energy multiple scatterings as well.
  
  Some comments now... Bucket (1) is lukewarm; we should only be interested in annual modulation for a dark matter hypothesis and there is no look-elsewhere effect. For buckets (2) and (3) let's look first at their Figure 4.
  
  
  
  
  Bucket (2) is room temperature. The phase of an annual modulation hypothesis is found to be inconsistent from the standard stationary halo expectation by 2.5σ. However, it is plain to see that it is consistent with the DAMA/LIBRA phase. If there is some bulk rotation/movement in the halo, perhaps this can be explained? Bucket (3) is certainly chilly, but there are two things to keep in mind. The amplitude is calculated for a particular model (WIMP-electron scattering with axial vector coupling), and the two experiments have very different targets (NaI crystal versus Xenon). Unfortunately we cannot compare apples with apples here and a conversion must take place, for which there is more information in a second XENON paper. For the last bucket let's look at their Figure 3.
  
  
  
  
  Bucket (4) is potentially large and freezing; a dark matter explanation should not induce an annual modulation in low energy multiple scatterings, and it appears to at 2.5σ. However, I can find nowhere in the paper where they quote the phase of this modulation! If indeed the phase is consistent with the single scattering phase, then this would be evidence for a background origin. Note that XENON100 is in Gran Sasso, as is DAMA/LIBRA, thus such a measurement would have implications for the DAMA/LIBRA result. So, XENON, what is the phase of the annual modulation in low energy multiple scatterings?

• Those following this blog will know we have been documenting somewhat the status of the galactic central excess of gamma rays seen in the Fermi data. The excess (over standard astrophysical backgrounds) is undeniably there, but the question of course to be answered is its origin: dark matter, or some (not yet fully understood) baryonic astrophysics? The most popular explanation in the latter set is by some population of millisecond pulsars (i.e. point sources) [see Sabine Hossenfelder's post here]. Recently, some-studies have analysed the Fermi data to see if the excess prefers a diffuse (e.g. dark matter) or point source origin. Both of the studies find a preference for a point source origin...
  
  This morning I stumbled upon a (days old) CERN Seminar from Tracy Slatyer, who may be in a unique position to comment on the issue, being an author of one of those new studies, as well as an author on one of the well cited papers arguing a dark matter interpretation. Below is the conclusion slide from the Slatyer talk, where it is interesting to see that the game has changed, with preference now for a point source origin over dark matter.
  
  
  
  
  

  This is science in action; it sounds like some very interesting new astrophysics will be revealed by the time the book is closed on this excess, and this should be celebrated.

• The PASCOS conference happened at ICTP at the end of last month; a very many interesting plenary talks (~30 mins each) are available as videos and worth a peruse.

• There's a public kaggle competition to design machine learning algorithms to best discover $\tau\to 3\mu$ decays at LHCb.

• Frank Wilczek's new book on beauty in nature is out. See Peter Woit's blog for a good summary and further links.

• Over the coming weeks, Stephen Hawking will be answering (some) submitted questions on artificial intelligence in a reddit AMA.

• In audio/video media:
• BBC documentary on dark matter, Dancing in the Dark. [50 mins]
• Pentaquarks on Sixty Symbols. [8 mins]
• A 20 minute short on SUSY and the LHC from Motherboard.
• A Brief History of Everything featuring Neil deGrasse Tyson at MinutePhysics. [8 mins]
• Physics Girl on New Horizons. [5 mins]
• Two bike-physics videos at MinutePhysics.
• How many chess games are possible? on Numberphile. [12 mins]
• Frank Wilczek talks on his new book at Google [40 mins], at the LSE [>1hr], or on radio [30 mins].

• The Kepler mission has discovered the first ~Earth-sized planet within the habitable zone of a Sun-like star [see xkcd]. There has been significant hype; for a no-nonsense take see Bad Astronomy. You can read the actual paper [pdf] here; they state, "The likelihood that this planet has a rocky composition lies between 49% and 62%."

• And while I was away, New Horizons flew by Pluto! In the tradition of ending each post with stunning shots of space, this probably takes the cake: the money shot in natural colour, a surface shot, and the farewell. Truly magnificent. (For more information, Nat Geo has a good story).
  
  
  
  

Friday wrap-up: first physics, CMS magnet, diboson excess...

Wherein I list some (mostly) recent happenings, ramble a bit, and provide links, in an order roughly determined by importance and relevance to particle physics. Views are my own. Content very definitely skewed by my own leanings and by papers getting coverage, and it may not even be correct. It is a blog after all...

Long time no blog -- turns out to be hard to keep up with everything in the midst of travel. So here are some "links without thinks" from the last month...

• As of June 3 the LHC has physics data being taken at 13 TeV! The live blog from the day can be found here [1 of 4], and there are five highlights videos on the CERN YouTube channel. The media hype for this milestone was significantly larger than for the first 13 TeV collisions 21 May. I wonder if this was a calculated decision from CERN... it seems that's what would have been preferred after the 7 TeV restart in 2010 (see Particle Fever e.g. 1:00:20).
  
  ATLAS made public a dijet animation and the below pp collision events with 17 vertices(!) from June 3...
  
  
  
  
  
• There was a rumour a couple of weeks back that the CMS magnet was in trouble. The earliest article I am aware of is this one (dated 8 June), which with the help of Google Translate suggests oil contamination of the liquid helium involved in cooling the 4T superconducting magnet. You can read the immediate response of CMS physicists here. Now CMS has released a statement (14 June), which is partly quoted below.
  
  CMS has been taking collision data since the 13TeV startup of the LHC on 3 June. During this period, the CMS magnet has been kept off due to an issue with the cooling system... The issue with the magnet cooling system was identified in the final preparatory phase leading to collisions in the LHC. While preparing for beam in CMS, a problem was found in the system that feeds liquid helium to the CMS superconducting magnet. The problem was diagnosed to be due to oil, which is used in the initial compression stages, reaching the so-called 'cold-box’ of the cryogenic system. The cold-box is a complex system with several sets of filters protecting three turbines along the path of the helium towards the magnet. In order to clean the oil contamination essentially all components of the cold-box have been extracted and replaced... CMS is confident that, following the LHC technical stop and the beam conditioning run that will start at the end of this week, after the low-intensity and commissioning period, the full magnetic field will be available for the 13 TeV LHC run.

• Certainly a month couldn't go by without another excess to keep people busy! Now it is in an ATLAS search for diboson resonances.
  
  
  
  
  The excess is of ~3σ local significance in WZ, WW and ZZ channels, and 2.5σ global. Jester has a write-up and some thoughts, and notes that there is a small excess at around the same mass scale in similar CMS searches. Along with the CMS WH resonance and right-handed W excesses at around 2 TeV, who knows, maybe there's life out there in the desert yet... and not too far away...
  
  
  
  (image credit John Pritchett)
  
  
  PS: I hear the ATLAS $h\to \mu\tau$ analysis is hopefully out within weeks (context)...

• The OPERA experiment has observed (i.e. at 5σ significance) tau neutrino appearance in a muon neutrino beam.

• A few more papers on standard-astrophysical-explanations for the galactic centre excess.

• The Perimeter Institute Convergence conference June 20-24 is archiving its very interesting talks here.

• The CMB@50 Conference held at Princeton from 10-12 June has been "storified" by Renée Hložek, links at her blog.

• Quantum Bayesianism (QBism) and Fuzzballs at Quanta.

• You can now visit more of CERN on Google Street View.

• In audio/video media:
• New podcast series from CERN: In Particular.
• A morning television spot for Australia's dark matter direct detection project. [6 minutes]
  

  On @sunriseon7 today 8.40am see #SUPL Oz's first #DarkMatter detector down a gold mine! Stream http://t.co/MKOaS3JDI3 pic.twitter.com/rlaOdXv857

  — Alan Duffy (@astroduff) June 6, 2015

• The CERN YouTube channel has been active, with videos on top physics, quark-gluon plasma, antimatter, SUSY, and the unknown...
• An interview with Steven Weinberg, drawing from his new book. [1 hour]
• SixtySymbols: what's left to do at the LHC? [9 minutes]
• Quanta Magazine have a YouTube channel now with some well-produced videos; here's their short on Freeman Dyson. [6 minutes]
• Conversations with an Astrophysicist: Dark Energy. [36 minutes]
• Numberphile: numbers with meaning. [13 minutes]

• In space news and pretty pictures:

• The Philae lander on Comet 67P/Churyumov-Gerasimenko is awake again, and ready to do science...
  
  
  
  
• ... Pluto and Charon in colour from New Horizons...
  
  
  
  
  (Only a couple of weeks now until closest approach: “Color observations are going to get much, much better, eventually resolving the surfaces of Charon and Pluto at scales of just kilometers.”)

• ... a flyover of Ceres from Dawn...
  

  Sit back, relax & fly over dwarf planet #Ceres. New video made w/ images taken during @NASA_Dawn's 1st mapping orbit https://t.co/Ys0WRVuPW4

  — NASA (@NASA) June 8, 2015

• ... and timelapses of the Earth from the ISS in HD...